Geometry Nodes. What to enter in the result to get a change in the Z axis?
I guess one solution to do what you are asking is a tree like the following:

Click to enlarge
The logic behind this is simple, use a  separate xyz node, and use a math&gt;add before the combine xyz node to control scale in the transform node.
The value node simply provides a means for uniform scaling in all or multiple axes.
This produces the following result on a point instance of cubes.

Do note that the geometry nodes project is a work in progress, and some features are still being adjusted as development continues.